1. Field of the Invention
The present invention relates to a ladder-type piezoelectric filter for use in the filter circuits of mobile radio communication units such as portable mobile radio communication units and cellular telephones, and more particularly, to a multielement-type piezoelectric filter well adapted for use in digital communication applications.
2. Description of the Related Art
Conventionally, a piezoelectric filter of this kind comprises unit filter circuits arranged in multiple stages and which are connected to an input terminal, an output terminal and a ground terminal. Each of the unit filter circuits includes a series resonator and a parallel resonator connected in an L-form. The series resonators have the same resonance frequency, while the parallel resonators have the same anti-resonance frequency. Also, the difference between the resonance frequency and the anti-resonance frequency is made constant. In order to improve the characteristics of a piezoelectric filter or to reduce the number of piezoelectric filters employed, various multielement-type piezoelectric filters have been proposed having a plurality of resonators.
According to a conventional configuration of the above-noted multielement-type piezoelectric filter, resonators and terminal boards are arranged in layers within a case. Accordingly, the number of components is relatively large, and assembly of the components is rather complex. To cope with a tendency in size reduction of mobile radio communication units, there is a need to reduce the height and size of such piezoelectric filters to dimensions on the order of millimeters. Furthermore, piezoelectric filters must be configured such that an electrical connection can be easily established between the piezoelectric filters and corresponding conductive paths on a printed wiring board. The piezoelectric filters must also be configured such that conductive paths other than terminals for connection with external conductive paths are not exposed in order to prevent electrical short circuits.
In order to reduce the overall thickness of a piezoelectric filter despite the use of a plurality of resonators, Japanese Patent Application Laid-Open (kokai) No. 8-8677 discloses the following configuration: a conduction circuit is formed on a connection substrate; a series resonator and a parallel resonator are mounted on the corresponding connection end portions of the conduction circuit such that lower electrodes of the resonators are connected to the corresponding connection end portions; upper electrodes of the resonators are interconnected by means of bonding wires; and a cap is attached onto the connection substrate. In such a configuration, conductive paths are exposed on the connection substrate, and thus satisfactory insulation protection is not established. Consequently, despite attachment of the cap onto the connection substrate, the bonding wire may contact the conductive path, or moisture may enter the interior of the cap to cause a short circuit. Furthermore, the use of bonding wires makes the connection work complex. Also, the connection substrate must have a region in which the resonators are disposed and a circuit region for establishing the required electrical connection without causing electrical interference with the resonators. Consequently, the plane area of the connection substrate becomes large with a resultant increase in filter size. Moreover, because the resonators are directly mounted on the conduction circuit, and the lower electrodes of the resonators and the corresponding conductive paths contact each other in a crossing manner, point contact cannot be established between the lower electrodes and the connection end portions of the conduction circuit. As a result, a favorable amplitude cannot be obtained.